1. Field of the Invention
The present invention relates to a pneumatic tire which can discharge a static electricity generated in a vehicle body and a tire, to a road surface, and a manufacturing method of the pneumatic tire.
2. Description of the Related Art
Recently, there have been proposed a pneumatic tire including a tread rubber blended with silica at a high ratio in order to reduce the rolling resistance that largely affects the fuel consumption of a vehicle and/or to increase braking performance (WET braking performance) on a wet road surface. However, compared to a tread rubber blended with carbon black at a high ratio, the electric resistance of such tread rubber is high, and accordingly, static charge generated on a vehicle body or the tire is prevented from being released to the road surface. As a result, problems like radio noises tend to occur.
Accordingly, there has been developed a pneumatic tire structured such that a tread rubber constructed by a nonconductive rubber blended with a silica or the like is provided with a conductive portion constructed by a conductive rubber blended with a carbon black or the like, whereby an electrical conduction performance can be achieved. For example, in a tire described in Japanese Unexamined Patent Publication No. 2010-115935, a conductive portion extending in a tire radial direction from a ground-contacting surface so as to reach a belt layer is provided in a tread rubber which is formed by a non-conductive rubber, thereby forming a conductive route for discharging a static electricity. However, in the tire, since the conductive route is formed via the belt layer, it can not correspond to the case that a topping rubber of the belt layer is formed by a non-conductive rubber.
On the other hand, in a pneumatic tire described in Japanese Unexamined Patent Publication No. 2009-126291, a tread rubber formed by a non-conductive rubber is provided with a conductive portion which extends to an inner side in a tire radial direction from a ground-contacting surface, extends only to one side in a tire width direction between a cap portion and a base portion, and is connected to a side wall rubber or a topping rubber in a carcass layer. However, since the conductive portion formed as an L-shaped cross sectional form as mentioned above is formed such a shape as to be one-sided in the tire width direction, there has been found that it causes a reduction of productivity and a deterioration of uniformity.
In other words, in order to make a profile of the tread rubber symmetrical in the tire having the conductive portion mentioned above, there is generated such a necessity that a thickness of a cap portion is differentiated between left and right sides by taking a thickness of the conductive portion into consideration, thereby causing a complication of a manufacturing step, and further a reduction of a productivity. Further, since a rigidity of the tread rubber is different between a side in which the conductive portion is provided and a side in which it is not provided, uniformity in a lateral direction of the tire tends to be deteriorated, and especially a lateral force variation (LFV) tends to become large. The same matter was indicated in Japanese Unexamined Patent Publication No. 2010-264920 by the same applicant as the Japanese Unexamined Patent Publication No. 2009-126291.
In addition, in the tire mentioned above, since an exposed position of the conductive portion in the ground-contacting surface exists in the vicinity of a tire equator, it tends to come to a covered state in which an upper end of the conductive portion to be exposed to the ground-contacting surface is covered by a thin skin of the non-conductive rubber. It is thought that this is caused by such a factor that the tread rubber gets together to the center portion in correspondence to a change of a cord angle of the belt layer due to a diameter expansion of the tire at a time of a cure molding. The tire which is manufactured while remaining in the covered state cannot achieve an electrical conduction performance in the early stage of a wear, which poses a problem.